As silicon film thickness of Silicon On Insulator (SOI) is reduced, for high-performance CMOS fabrication, it becomes necessary to increase the thickness of source/drain regions above the SOI thickness. This results from the fact that, as the SOI film becomes thinner, a reduced amount of Si material is available from which to form silicide for source/drain contacts. Additionally, thinner source/drain regions can degrade on-current due to increased series resistance.
To maintain or reduce overall source/drain series resistance, including the silicide contact resistance, techniques have emerged to form raised source/drain (RSD) structures. For example, selective epitaxial growth of silicon has been used to grow silicon on the source/drain and the top of poly gates, leaving no silicon on gate sidewall spacers. One problem with this technique is that epitaxial growth must take place at temperatures as high as 750° C., which may cause significant transient enhanced diffusion of dopants. The unnecessary dopant redistribution in halo/extension regions degrades the performance of devices and short channel immunity. Moreover, with a very thin SOI film, it can be difficult to form epitaxial silicon without causing silicon agglomeration due to possible contamination of carbon in certain chemical vapor deposition tools.
Another technique involves SiGe selective epitaxial growth on source/drain regions. Advantageously, this technique eliminates transient enhanced diffusion of dopants which may otherwise occur at high temperatures. Unfortunately, however, Ge degrades silicide contact formation and contact resistance due to its inherent function as a diffusion barrier. To reduce such undesirable effects, the surface of SiGe may be capped with additional epitaxial silicon, using a high temperature process, which may cause transient enhanced diffusion.
Another problem with raised silicon/silicon germanium source and drain regions involves contact formation. High quality contacts to silicon-based field effect transistors are typically achieved through a silicide process, where a metal such as cobalt or titanium is alloyed with silicon to form the contact. However, this process is generally not as effective with semiconductor materials other than silicon. For example, a cobalt silicide contact formed to a source disposed in a strained Si layer and an underlying SiGe layer may form a high-resistivity compound in the SiGe layer, thereby compromising the functionality of the contact.
The invention is directed to overcoming one or more of the problems as set forth above.